This invention relates to solid state electrical controls systems and more particularly to a control system wherein the control system is isolated from the high voltage AC lines. In controlling the operation or conductivity of a semiconductor switching device, for example, one connected to a relatively high voltage supply source, control signals are often developed in a circuit operating on a relatively low voltage, such as conventional logic circuits. There is difficulty in providing circuitry that is sensitive to the conditions of the power supply and providing control signals which will interrupt the faulted power supply in response to a sensed signal and at the same time maintain it isolated from the high AC voltage being controlled.